U.S. patent number 10,564,505 [Application Number 15/925,597] was granted by the patent office on 2020-02-18 for graphene as an alignment layer and electrode for liquid crystal devices.
This patent grant is currently assigned to The Government of the United States of America, as represented by the Secretary of the Navy. The grantee listed for this patent is The Government of the United States of America, as represented by the Secretary of the Navy, The Government of the United States of America, as represented by the Secretary of the Navy. Invention is credited to Rajratan Basu, Jesse A. Frantz, Daniel Kinnamon, Jakub Kolacz, Jason D. Myers, Christopher M. Spillmann.
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United States Patent |
10,564,505 |
Basu , et al. |
February 18, 2020 |
Graphene as an alignment layer and electrode for liquid crystal
devices
Abstract
A graphene and liquid crystal device comprising a substrate, a
layer of graphene on the substrate, and a layer of liquid crystal
on the layer of graphene. A method of making a graphene and liquid
crystal device comprising the steps of providing a substrate,
depositing a layer of graphene on the substrate, and depositing a
layer of liquid crystals on the layer of graphene.
Inventors: |
Basu; Rajratan (Annapolis,
MD), Kinnamon; Daniel (Annapolis, MD), Spillmann;
Christopher M. (Annandale, VA), Kolacz; Jakub
(Washington, DC), Frantz; Jesse A. (Washington, DC),
Myers; Jason D. (Alexandria, VA) |
Applicant: |
Name |
City |
State |
Country |
Type |
The Government of the United States of America, as represented by
the Secretary of the Navy |
Arlington |
VA |
US |
|
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Assignee: |
The Government of the United States
of America, as represented by the Secretary of the Navy
(Washington, DC)
|
Family
ID: |
63669366 |
Appl.
No.: |
15/925,597 |
Filed: |
March 19, 2018 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20180284518 A1 |
Oct 4, 2018 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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62478929 |
Mar 30, 2017 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02F
1/133753 (20130101); G02F 1/1337 (20130101); G02F
1/1418 (20130101); G02F 1/13439 (20130101); G02F
1/133711 (20130101); G02F 2001/133757 (20130101); G02F
2001/133738 (20130101); G02F 2001/133796 (20130101) |
Current International
Class: |
G02F
1/141 (20060101); G02F 1/1337 (20060101) |
Foreign Patent Documents
Other References
Basu, Rajratan, Daniel Kinnamon, and Alfred Garvey. "Graphene and
liquid crystal mediated interactions." Liquid Crystals 43.13-15
(2016): 2375-2390. cited by applicant .
Basu, Rajratan, and Samuel A. Shalov. "Graphene as transmissive
electrodes and aligning layers for liquid-crystal-based
electro-optic devices." Physical Review E 96.1 (2017): 012702.
cited by applicant.
|
Primary Examiner: Nguyen; Lauren
Attorney, Agent or Firm: US Naval Research Laboratory
Hunnius; Stephen T.
Parent Case Text
REFERENCE TO RELATED APPLICATION
This application is a non-provisional of, and claims priority to
and the benefits of, U.S. Provisional Patent Application No.
62/478,929 filed on Mar. 30, 2017, the entirety of which is hereby
incorporated by reference.
Claims
What we claim is:
1. A method of making a graphene and liquid crystal device
comprising the steps of: providing a substrate; depositing a layer
of graphene on the substrate; and depositing a layer of liquid
crystals on the layer of graphene heating the graphene and liquid
crystal device; cooling the graphene and liquid crystal device;
wherein the layer of liquid crystals comprised liquid crystals in
the isotropic phase before the step of heating the graphene and
liquid crystal device, and wherein the layer of liquid crystals
comprised liquid crystals in the nematic phase after the step of
cooling the graphene and liquid crystal device.
2. The method of making a graphene and liquid crystal device of
claim 1 further comprising the steps of: utilizing the layer of
graphene as both an alignment layer and an electrode for a liquid
crystal device.
3. The method of making a graphene and liquid crystal device of
claim 2 further comprising the steps of: applying a voltage to the
layer of graphene; and aligning the molecules of the layer of
liquid crystals.
4. The method of making a graphene and liquid crystal device of
claim 1 further comprising the steps of: inducing the .pi.-.pi.
electron stacking; aligning the molecules of the layer of liquid
crystals and the layer of graphene; and achieving a uniform planar
aligned state of the molecules of the layer of liquid crystals.
5. The method of making a graphene and liquid crystal device of
claim 1 wherein the layer of liquid crystals comprised liquid
crystals in the isotropic phase before the step of heating the
graphene and liquid crystal device, and wherein the layer of liquid
crystals comprised liquid crystals in the smectic-A* and smectic-C*
phase after the step of cooling the graphene and liquid crystal
device.
6. The method of making a graphene and liquid crystal device of
claim 1 further comprising the steps of: utilizing the layer of
graphene as both an alignment layer and an electrode for a liquid
crystal device.
7. The method of making a graphene and liquid crystal device of
claim 1 further comprising the steps of: applying a voltage to the
layer of graphene; and aligning the molecules of the layer of
liquid crystals.
8. The method of making a graphene and liquid crystal device of
claim 5 further comprising the steps of: inducing the .pi.-.pi.
electron stacking; aligning the molecules of the layer of liquid
crystals and the layer of graphene; and achieving a uniform planar
aligned state of the molecules of the layer of liquid crystals.
Description
BACKGROUND
This disclosure teaches methods for achieving alignment of liquid
crystal (LC) using graphene layers.
This disclosure teaches concurrent creation of alignment layers and
electrodes using graphene for controlled reorientation of LC by an
electric field.
This disclosure teaches a new product comprising graphene layers as
both alignment layers and electrodes.
Electro-optic LC devices can be transmissive (for example,
displays), reflective (for example, tunable filters), or refractive
(for example, waveguides such as steerable electro-evanescent
optical refraction devices).
The principle mechanism of these devices relies on two factors: the
ability of the surface to homogeneously align liquid crystals and
the ability of that alignment to change with an applied electric
field.
These two requirements are usually fulfilled by electrodes with
relatively low transmission losses that are deposited on parallel
substrates and a LC aligning layer between the electrodes and the
LC.
In a conventional LC cell, indium tin oxide layers serve as the
electrodes and polyimide layers with unidirectional rubbing
direction serve as the aligning layers of the LC director at the
two substrates. A wide range of conductive materials and
(in)organic materials have been developed and demonstrated as
effective electrodes and alignment layers for LC, respectively.
While these conductive and alignment layer combinations work for
the majority of LC-based transmissive applications due to short
path lengths (.about..mu.ms), an ideal scenario is where the two
functions are met with a single layer of material.
In certain architectures, such as a slab waveguide with an
LC-cladding, additional layers of material introduce unwanted
absorption and scattering losses over relatively long path lengths
(.about.cms), thereby significantly degrading throughput.
Reducing the conductive and aligning functions with a single layer
of material offers the potential to minimize these losses and
optimize throughput of light over a wide range of spectral bands.
Graphene, the two-dimensional form of carbon, is an ideal material
to fulfill the role as both an LC electrode material and an
aligning layer.
We demonstrate the first use of graphene being concurrently used as
the electrode and the alignment layer.
SUMMARY OF DISCLOSURE
Description
This disclosure teaches methods for achieving alignment of liquid
crystal (LC) using graphene layers.
This disclosure teaches concurrent creation of alignment layers and
electrodes using graphene for controlled reorientation of LC by an
electric field.
This disclosure teaches a new product comprising graphene layers as
both alignment layers and electrodes.
We demonstrate the first use of graphene being concurrently used as
the electrode and the alignment layer.
DESCRIPTION OF THE DRAWINGS
The following description and drawings set forth certain
illustrative implementations of the disclosure in detail, which are
indicative of several exemplary ways in which the various
principles of the disclosure may be carried out. The illustrated
examples, however, are not exhaustive of the many possible
embodiments of the disclosure. Other objects, advantages and novel
features of the disclosure will be set forth in the following
detailed description when considered in conjunction with the
drawings.
FIG. 1 is a schematic diagram of one embodiment of a LC device with
a graphene alignment layer.
FIG. 2 illustrates several views of Crossed Polarized Light
Microscopy (XPLM) images of a thin layer of LC on a CVD grown
graphene film on copper foil at 0.degree., 45.degree. and
90.degree. to the polarizer. Domains 1 and 2 are labeled in order
to track the change in intensity as the sample is rotated. One view
illustrates Normalized intensity as a function of angle of rotation
for domains 1 and 2. The final view is a schematic representation
of LC stacking on polydomain graphene, highlighting the existence
of domain boundaries. The .pi.-.pi. electron stacking is
demonstrated by matching the benzene rings of the LC molecules on
the honeycomb graphene surface. The orientation of the LC on the
graphene domains determines which rotation angle will show bright
and dark states under XPLM.
FIG. 3 illustrates several views. One view is a CVD-grown monolayer
graphene film that has been transferred from the copper foil to the
glass substrate. XPLM images of the smectic-A* phase on a monolayer
graphene film on a glass substrate under a crossed polarised
microscope. XPLM image of the smectic-C* phase on a monolayer
graphene film on a glass substrate under a crossed polarised
microscope. Microphotographs of the smectic-C* phase at three
different relative angles, 0.degree., 45.degree. and 90.degree.,
respectively, under a crossed polarised microscope. The domains'
transition from dark to bright at every 45.degree. rotation
confirms aplanar alignment of the smectic-C* on graphene. Also
illustrated is a Natural smecticC* phase. The final view
illustrates Graphene stabilised smectic-C* phase schematic
illustration of planar alignment.
FIG. 4 illustrates several views. A LC device created using two
graphene coated substrates, which are used as both the alignment
layers and electrodes. XPLM image of the cell at 45.degree. with
respect to the polarizer and analyzer shows a bright state. XPLM
image of the cell aligned along the polarizer axis shows a dark
state. This indicates planar alignment of the LC is induced by the
graphene surface. The final view is a Schematic drawing of the LC
aligned in the device based on the XPLM images.
FIG. 5 illustrates several views. XPLM images of the LC in the
graphene-based cell with no voltage, 15V, 30V and 45V. The
corresponding schematics illustrate how the LC director starts to
align with the electric field as the voltage increases, obtaining a
homeotropic alignment at 45 V.
FIG. 6 illustrates a Schematic diagram of graphene electrodes being
using in a LC cladded waveguide.
DETAILED DESCRIPTION OF THE INVENTION
This disclosure teaches methods for achieving alignment of liquid
crystal (LC) using graphene layers, teaches concurrent creation of
alignment layers and electrodes using graphene for controlled
reorientation of LC by an electric field, and teaches a new product
comprising graphene layers as both alignment layers and
electrodes.
We demonstrate the first use of graphene being concurrently used as
the electrode and the alignment layer.
This innovation is unique because LC between two graphene sheets
forms a natural electro-optic device. LC molecules can stabilize
themselves on the honeycomb pattern of graphene or carbon
nanotubes, employing the .pi.-.pi. electron stacking with a binding
energy of -2 eV.
This replacement effectively reduces the thickness of all the
alignment layers and electrodes from 10-100 s of nm to about 1
nm.
The reduction in path length has the potential to achieve higher
optical throughput and access a wider spectral range for
electro-optic applications.
The stacking of benzene on graphene also enhances orientational
order. This order enhancement coupled with the relatively large
anchoring strength that results from .pi.-.pi. stacking of the LC
on the graphene honeycomb means lower thermal scattering losses
from the LC, which make it more practical for NIR-vis-UV light in
waveguide architectures.
Example 1
Nematic Alignment on Graphene
In one embodiment of the invention, shown in shown in FIG. 1, a LC
device is comprised of a substrate, a monolayer of graphene and a
LC thin film. The alignment of this embodiment was tested in a
prototype.
Chemical vapor deposition (CVD) grown monolayer graphene film on a
copper foil was first obtained from Graphene Supermarket, Inc. The
graphene film on copper was continuous, with irregular holes and
cracks. In addition, the graphene film was polycrystalline (i.e.
the presence of grains with different crystallographic
orientation).
A small droplet of the LC in the isotropic phase was first placed
on the graphene film. The LC droplet was then blown away gently by
a dust blower, which left a thin LC layer at the graphene surface.
The LC-coated graphene on copper substrate was then heated up in
the isotropic phase to get rid of any residual order from the
coating process and then slowly cooled down to the nematic
phase.
The alignment of the LC on the graphene film was studied by
reflected Cross Polarized Light Microscopy (XPLM) and the results
are presented in FIG. 2. In XPLM, a sample is placed between two
crossed polarizers and rotated to observe areas of highest
intensity (a bright state) and the lowest intensity (a dark
state).
In LC, a dark state is observed when the nematic director ft is
parallel to the polarizer (or to the crossed analyzer). The bright
state appears when ft is at 45.degree. with respect to the
polarizer (or crossed analyzer). It is worth mentioning that the
bare monolayer graphene film on copper foil appears completely
dark.
After coating a thin LC layer on the graphene film, different
crystallographic graphene domains with grain boundaries are clearly
visible in the XPLM images shown in FIG. 2. Three domains: 1, 2 and
3 are labelled in FIG. 2 and their intensities were tracked as the
sample was rotated under reflected XPLM. FIG. 2 shows that after
rotating 45.degree., domain 1 becomes bright, domain 2 becomes dark
and domain 3 becomes bright. FIG. 2 shows that after rotating
45.degree. more (i.e. a total of 90.degree. from the initial
state), domain 1 becomes dark, domain 2 becomes bright and domain 3
becomes dark. FIG. 2 shows the normalized domain intensity of
domains 1 and 2 as a function of the relative angle of
rotation.
The .pi.-.pi. stacking interaction is schematically illustrated in
FIG. 2 by matching the LC's benzene rings on the graphene-honeycomb
structure. Two possible LC domains on graphene are schematically
presented in FIG. 2, and their respective dark or bright states are
demonstrated by showing the director ({circumflex over (n)})
orientation with respect to the polarizer and analyzer.
These results suggest that the LC on graphene can achieve a uniform
planar aligned state, which can transit from dark to bright when
rotated 45.degree..
This uniform planar aligned state is induced by the strong
.pi.-.pi. electron stacking.
Example 2
Smectic Alignment on Graphene
In a second prototype, ferroelectric LCs in the smectic-A* and
smectic-C* were used as the liquid crystal. The monolayer graphene
film was first grown by CVD processing onto a copper foil, then
transferred onto glass. The glass substrate with graphene film was
first placed on a hot plate at 110.degree. C. A small droplet of
ferroelectric liquid crystal (FLC) MX40636 (LC Vision, LLC, cooling
phase sequence: Iso-97.degree. C.-N*-82.degree.
C.-smectic-A*-76.degree. C.-smectic-C*--10.degree. C. crystal) in
the isotropic phase was placed on the graphene film. The LC droplet
(in the isotropic phase) was then blown away gently by a dust
blower, which left a thin LC layer in the isotropic phase at the
graphene surface.
The LC-coated graphene on glass substrate was then slowly cooled
down to the smectic-A* and then to the smectic-C* phase,
respectively. The FLC MX40636-coated graphene film on glass
substrate was then studied using XPLM.
FIG. 3 shows XPLM images of the smectic-A* and smectic-C* phases on
graphene. Polycrystalline graphene grains are clearly visible from
the dark and bright LC domains for both the phases.
FIG. 3 represents the smectic-C* phase on graphene at three
relative angles: 0.degree., 45.degree. and 90.degree. to the
crossed-polarizers. A domain's transition from dark to bright (or
bright to dark) at every 45.degree. rotation confirms that graphene
imposes planar alignment on the smectic-C* phase due to the strong
.pi.-.pi. electron stacking. FIG. 3 schematically shows a natural
smectic-C* phase. FIG. 3 schematically represents a
graphene-stabilized smectic-C* order.
Example 3
LC-Graphene Electro-Optic Device
In another embodiment, shown in FIG. 4, an electro-optic LC device
is comprised of a substrate with a graphene layer, a second
substrate with a graphene layer, two Mylar spacers to hold the
surfaces apart and a LC layer to fill in the cavity between
graphene layers. This embodiment was tested in a prototype.
Two glass substrates with monolayer graphene film were used to
prepare a cell (the monolayer graphene film was first grown by CVD
processing onto a copper foil, then transferred onto glass). The
glass substrates with graphene were placed together to make a cell
with an average thickness of 12 .mu.m. The graphene-based LC cell
was then filled with the LC mixture E7.
The planar alignment of the LC in the cell was studied using XPLM
(FIG. 4). Note that FIG. 4 shows a XPLM image of a bright region of
the cell, which becomes dark when rotated by 45.degree.. This
confirms a planar alignment of the LC director in the cell. The
cell was then rotated back to 0.degree. relative position at its
bright state and a voltage was applied across the cell using the
graphene as electrodes.
FIG. 5 shows the XPLM images of the cell at three different
voltages: 15V, 30V and 45V, respectively. When the voltage
increases across the cell, the LC director starts to reorient along
the electric field, as schematically shown in FIG. 5. The cell
shows a dark XPLM texture at 45V, indicating a homeotropic
alignment of the LC director in the graphene-based LC cell. These
results, thus, indicate that the graphene film can act both as the
alignment layer and the electrodes in a graphene-based LC cell.
Example 4
LC-Graphene Cladded Waveguide
In another embodiment, shown in FIG. 6, graphene is used as an
alignment layer and electrode for a LC cladded waveguide. In this
architecture, the effective index of the propagation mode can be
changed by applying a field across the graphene electrodes and
inducing a reorientation of the LC.
This invention demonstrates the utility of graphene as a LC
alignment layer.
The invention demonstrates that a monolayer of graphene can replace
both the electrode and alignment layer in LC devices.
The invention enables a reduced path length through absorbing and
scattering layers, extending the use of LC devices into spectral
bands that were previously not feasible due to absorption and
scattering of alignment layers and electrodes. These spectral bands
may include, but are not limited to ultraviolet, visible, near-,
short-wave, mid-wave and long-wave infrared.
This graphene alignment layer/electrode can be used for any
LC-based electro-optic device including, but not limited to
displays, polarization gratings, tunable filters, refractive
optical waveguides.
The single conductive/alignment layer is not necessarily limited to
graphene, but includes any other two-dimensional material with
electrically conductive properties and a propensity to align LC
materials and mixtures.
The single conductive/alignment layer, such as graphene, may be
chemically modified directly to promote uniaxial LC alignment on
the surface.
Uniaxial alignment of LC on graphene may be promoted by adding an
additional photoalignment step, whereby polarized light is used to
absorb or chemically bond LC-compatible molecules to the graphene
surface.
Uniaxial alignment of LC on graphene may be promoted by non-surface
means including, but not limited to flow alignment,
photo-orientation and slow cooling from the isotropic phase.
Other embodiments include but at not limited to the LC layer
comprised of a single molecular component or mixtures of components
with a liquid crystal phase, including but not limited to nematic,
cholesteric, smectic and discotic phases.
The LC may possess a positive or negative dielectric anisotropy and
a permanent molecular dipole to facilitate molecular switching in
response to an applied voltage
Furthermore, the graphene can be multi-layered and the device
substrate may be rigid or flexible.
With this invention, the graphene can be a liquid crystal alignment
layer. The graphene can be an electrode for switching the
orientation of a liquid crystal.
Taught herein is simultaneous demonstration of graphene as both an
alignment layer and electrode. The embodiments taught herein reduce
the gap between electrodes, and reduce voltage amplitudes.
A reduced path length (carbon atom-thick) allows for reduced
absorption and scattering contributions over a wide spectral range.
The examples taught herein are compatible with alignment of
different LC phases (i.e. nematic, smetic).
These embodiments open the utility of LC-based electro-optic
devices beyond limited spectral bandwidth of current alignment
layers (i.e. polyimide) and electrodes (i.e. indium tin oxide,
ITO).
Here, graphene provides a means to both align LC and apply a
voltage in devices with a single carbon atom-thick layer.
The examples herein demonstrate the ability to minimize the number
of layers in the device, simplify the fabrication process, have
reduced optical path length and optimize the transmission.
Still furthermore, the examples demonstrate lower applied voltages
and greater resistance to device degradation.
The above examples are merely illustrative of several possible
embodiments of various aspects of the present disclosure, wherein
equivalent alterations and/or modifications will occur to others
skilled in the art upon reading and understanding this
specification and the annexed drawings. In addition, although a
particular feature of the disclosure may have been illustrated
and/or described with respect to only one of several
implementations, such feature may be combined with one or more
other features of the other implementations as may be desired and
advantageous for any given or particular application. Also, to the
extent that the terms "including", "includes", "having", "has",
"with", or variants thereof are used in the detailed description
and/or in the claims, such terms are intended to be inclusive in a
manner similar to the term "comprising".
* * * * *